The following are recommended changes to the existing
Over-Current Protection Pickup Guidelines:
Time Delay
Phase Pickup
Instantaneous
Phase Pickup
1. The
minimum line to ground (LG) fault current will be calculated using a 10 Ohm
fault impedance.
2. The
minimum phase fault current will be calculated using a 2 ohm fault impedance
and will be the lesser of the phase to phase (LL) and a three phase (3p) fault.
(2 ohms is the approximate arc impedance through air for our standard wire
spacing)
3. At
a backup device (recloser or circuit breaker) we should strive (i.e. not
mandatory) to detect a bolted fault at the end of the next device’s zone. This
applies for both ground and phase settings.
Minimum =
line ampacity or 2 x (maximum load current) which ever is the lowest
Maximum = 1
phase (EOZ) 2 ohm (the lesser of the phase to phase and three phase fault, at
the End OF ZONE, through a two Ohm impedance)
Max. Backup =
1-LL (ENZ) (the bolted phase to phase fault, at the END of the Next Zone)
Ground Pickup
Minimum =
0.3-0.5 x (maximum load current) or 0.3-0.5 x (line ampacity) which ever is the
lowest
Maximum =
1-LG (EOZ) 10 Ohm (the line to ground fault current, at the End Of Zone,
through a ten Ohm impedance)
Max. Backup =
1-LG (ENZ - the bolted line to ground fault, at the End of th Next Zone)
Minimum =
0.9 x 1-LL (EOZ)
Maximum =
1.25 x 3phase bus fault
Ground Pickup
Minimum =
0.9 x 1-LG (EOZ)
Maximum =
1.25 x 3phase bus fault
Recloser with Phase and Ground Settings
Phase Pickup
Minimum =
2 x (maximum load current)
Maximum = 1
phase (EOZ with 2 Ohm fault impedance)
Max. Backup =
1-LL (ENZ)
Minimum =
0.5 x (maximum load current)
Maximum =
1-LG (EOZ with 10 Ohm fault impedance)
Max Backup =
1-LG (ENZ)
Phase Pickup
Minimum =
2 x (maximum load current)
Maximum =
1-LG (EOZ with 10 Ohm fault impedance)
Max. Backup =
1-LG (ENZ)
The value of a (10) ten ohm ground fault impedance was
initially chosen because it appeared to be the most prevalent in the industry
from what could be determined. It is recognized that fault impedance from zero
to forty Ohms are in use. The (10) ten ohm value gained legitimacy after
reviewing EPRI Report EL-3085, Distribution Fault Current Analysis. This report
indicates that 83% of faults involved the neutral or ground. The maximum fault
impedance was calculated to be 3 Ohms.
As stated earlier, the value of a (2) two ohm phase fault
impedance was chosen because it is the approximate arc impedance through air
for our standard wire spacing.
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